Perfusion CT examinations for diagnosing an ischemic stroke are known. This examination method allows a quantitative determination of the brain perfusion and so regions of the brain with insufficient perfusion can be detected directly, and a distinction can be made between the already irreversibly damaged cerebral matter, the infarct core, and the only reversibly damaged cerebral matter, the infarct penumbra.
In order to assess the blood flow in the brain, a brief contrast agent bolus is administered intravenously during a perfusion CT examination and a number of CT records are generated at defined time intervals. The principle of this examination method consists of the X-ray density of the brain briefly increasing as a result of the bolus-type administration of an intravenous contrast agent. The extent and the time profile of this increase in density, which can be detected in a CT examination, allow conclusions to be drawn about the cerebral perfusion. Parameters which describe the perfusion in the brain are calculated using different mathematical algorithms and methods. These can be illustrated in color-coded parameter images.
The contrast agent is distributed to different extents in the different regions of the brain as a result of the natural differences between gray matter and white matter, and therefore an interpretation of the parameter images which is differentiated according to tissue type is possible. Gray matter combines the regions of the central nervous system which mainly comprise neuron bodies. The totality of nerve fibers form the white matter. In the brain, most parts of the white matter, that is to say in the cerebrum and the cerebellum, are surrounded by the gray matter.
The most important parameters are the cerebral blood flow, the cerebral blood volume and parameters for describing the delay in the contrast agent distribution. The cerebral blood flow forms the basis of the oxygen and nutrient supply for all neurons in the brain. In the case of a healthy adult, approximately 15% of the cardiac output flows through the brain and the surrounding tissue thereof; this corresponds to approximately 700 ml blood per minute. The cerebral blood flow therefore specifies the volume of blood (ml) flowing per mass of tissue (g) per unit time (min). The cerebral blood flow exhibits noticeable regional differences in the brain. In the white matter, the value of the cerebral blood flow is approximately a third of what it is in the gray matter. Due to a lack of energy, the synaptic function of the neurons is disturbed below a certain value and there are failures in the brain. These failures are completely reversible as long as the perfusion normalizes again. However, if the blood flow continues to drop, the structural metabolism in the neurons is also disturbed and there is irreversible damage to the tissue in the case of an extended period of undersupply.
After a stroke, a peripheral edge of only reversibly damaged cerebral matter—the ischemic infarct penumbra—often forms around the irreversibly damaged infarct core; the cells of said penumbra no longer functioning neurologically but not yet being irreversibly damaged. Irreversible damage to the penumbra only occurs once the undersupply lasts. It is for this reason that the therapy after a stroke concentrates on restoring the perfusion in these regions in order to limit the damage to the brain to the greatest possible extent.
The amount of blood located within the brain for supplying the brain and the meninx at a given time is referred to as the cerebral blood volume. Thus, it specifies what volume of blood (ml) can be found per mass of tissue (g). The ratio of the blood volume in the white matter to that in the gray matter is approximately one to two.
Parameters characterizing a perfusion delay within the brain include, for example, the mean transit time and the time a contrast agent bolus requires to reach maximum enrichment in a certain tissue region. This is also called the time until maximum hyperdensity. The mean transit time specifies the amount of time a contrast agent bolus requires to pass through the tissue of interest from a supplying artery and into a venous vessel. Both parameters react very sensitively to variations in the blood supply.
A problem with the previous diagnosis of ischemic cerebral matter lies in the fact that the white matter in the healthy tissue has smaller values of cerebral blood flow and cerebral blood volume than healthy gray matter, just like ischemic regions. It is for this reason that it has not previously been possible for an unambiguous distinction to be made between the healthy white matter and the ischemic cerebral matter belonging to the penumbra. This makes automatic quantitative determination of the penumbra more difficult and the therapy for restoring the perfusion cannot be applied to this region in a targeted fashion.
Thus, it has not previously been possible for a distinction to be made between the types of tissue in a perfusion CT examination, that is to say it has not been possible for the regions undersupplied with blood to be isolated automatically. So as to nevertheless make an evaluation possible, the treating medical practitioner previously marked a region of interest in the anatomically relevant regions of the parameter images. Here, the manual marking of the ischemic region is not always complete and correct; it relies heavily on the experience of the treating medical practitioner; and it is furthermore impeded by the fact that the gray matter and the white matter are strongly intermeshed. Thus, it is practically impossible to differentiate between the white matter and the gray matter, even with the aid of this manual intervention.